Interpretation of pleural fluid

What it is

Pleural effusions are divided into transudates and exudates, and the division decides how much work follows. Transudates are due to an imbalance between hydrostatic and oncotic pressures that increases fluid movement across the capillaries into the visceral pleura and the pleural space; such fluid requires no further intervention except treatment directed at the underlying disease. Exudative effusions are due to pleural and lung inflammation resulting in increased capillary and pleural membrane permeability, and they require more extensive diagnostic investigation.

How it is diagnosed

The Light criteria are three, and an exudate is fluid that meets at least one of them:

1. Pleural fluid protein to serum protein ratio above 0.5
2. Pleural fluid LDH to serum LDH ratio above 0.6
3. Pleural fluid LDH above 2/3 of the upper limit of normal for serum LDH

One of the three is enough; they are not counted together. That is why the sample is always sent with a serum sample beside it. A worked panel sets the two side by side: pleural fluid LDH 315 U/L against serum 200 U/L, pleural protein 55 g/L against serum 70 g/L, pleural glucose 3.0 mmol/L against serum 6.2 mmol/L, with a cell count reported as lymphocytes dominant. Glucose is read next. A low pleural glucose also indicates an exudative effusion. A pleural fluid glucose below 60 mg/dL is usually due to rheumatoid pleurisy, complicated parapneumonic effusion or empyema, malignant effusion, tuberculous pleurisy, lupus pleuritis, or esophageal rupture. A pleural glucose below 30 mg/dL in particular suggests an empyema or a rheumatic effusion.

Where tuberculosis is suspected the fluid is asked a different question. Adenosine deaminase in pleural fluid, and to a lesser extent in cerebrospinal fluid, may assist in confirming suspected tuberculosis. But fluid alone is a weak witness: in extrapulmonary disease the yield of fluid examination, whether cerebrospinal, ascitic, pleural, pericardial or joint, is classically very low, because there are generally fewer organisms, particularly in meningeal or pleural fluid. So culture or histopathology of tissue is more important there, and the diagnosis is more challenging than in the pulmonary form.